The present invention relates to static structures, buildings, dwellings and the like and more particularly to earth sheltered structures.
Due to increased energy costs, desire for conservation and increased interest in designing structures which make more complete use of and merge into the building site, there has been increased awareness of earth sheltered or underground structures. In an earth sheltered building, the soil is used as a barrier against air filtration, and the heat contained in the soil assists in heating and cooling. Earth sheltering reduces energy consumption and moderates temperature changes. Covering or surrounding a structure with earth has other advantages. These include increased protection from vandalism, break-ins and the like, a reduction in noise infiltration or interior noise levels due to exterior sources and increased protection to the occupants from earthquakes, tornados, winds and the elements.
Many problems are, however, associated with earth sheltering designs. These problems include those which are found in conventional dwelling construction and others. Such other problems relate to infiltration, thermal breaks and leaks, freeze/thaw cycles, moisture and water penetration, ventilation, lighting and the like. Also, due to the increased loads on the structures from the soil, additional structural problems must be solved.
Various approaches have been used in the past in constructing earth sheltered dwellings. These approaches include more or less conventional post and beam structures, dome structures and arch structures. A typical post and beam design incorporates buried walls and flat roofs which are covered with soil. Various materials may be used in such structures, including reinforced concrete, masonry and wood. Problems may be presented with each of these building materials. Typically, due to the loading of the flat roof structure, special reinforced beams, joints and/or steel members must be used. Generally, to avoid failure of the roof structure, the spans must be shorter than with conventional housing or greater strength, and more expensive steel beams must be used.
Due to these problems with conventional post and beam approaches, the dome structure and arch structure have been used. Such structures permit an increase in earth loads and therefore increased thermal efficiency when compared to the post and beam structure. A dome structure resolves loading essentially in all directions. A dome, under load, acts similar to a diaphragm. It balances the loads applied to it and obtains high efficiency. Problems are, however, presented with a dome structure in earth sheltered dwellings. The dome is an essentially closed structure. In order to provide ventilation, light, access and the like, the exterior surface of the dome must be penetrated. These penetrations must be reinforced or specially adapted to handle such penetrations. Further, the interior floor plan layout of a dome structure is more complicated when compared to conventional construction. Everytime an exterior wall is joined, a curved surface is presented. In general, contractors find it easier to work with right angles.
Many of the problems associated with post and beam and dome structures are eliminated with an arch structure. An arch supports itself by resolving loads from the roof and walls without creating tension in the structure. The pressure on the outside of the structure helps to maintain its integrity since it is being compressed. An arch is open at both ends. This provides access, egress, ventilation and light. Such openendedness essentially eliminates the need for penetration of the arch structure and the resulting additional expense of compression rings. An arch structure also reduces the complexity of the floor plan layout when compared to dome structures. More right angle work is obtainable.
In constructing an arch based earth sheltered dwelling, various structural considerations must be kept in mind. The loads on an arch essentially create four different kinds of stresses which must be resisted. These are bending stresses, shear stresses, thrust stresses and compressive stresses.
Bending stress tends to cause the top of the arch to move inward and the sides of the arch to move outward. Bending stresses are generally significant only during the construction phase when the earth cover is being placed on the arch. When covered with earth, the arch is stable. The earth at the sides of the structure resists the tendency of the arch to move outward of the sides and prevents the arch from moving downward at the top.
Shear stresses tend to cause the arch to break. Thrust stresses tend to cause the base of the arch to spread. Compressive forces tend to crush the ends of the arch where it joins the footings. Shear stresses are significant after earth load is in place on the top and sides of the arch. Thrust is a lateral tension stress which is located at the base of the arch. This is caused by the arch shape itself. Compressive stresses are maximum under final load conditions. These occur where the arch joins the footings.
One approach to accommodate the structural considerations of an arch involves forming the arch from a plurality of structural steel panels which are preformed and joined on site. The panels are joined in a plurality of rows to obtain the desired circumferential dimension and longitudinal dimension or distance between the front and back of the arch. The steel panels are then sprayed with a concrete mix, such as a Shotcrete or Gunite mix. The concrete mix is sprayed onto the structural steel. The concrete and steel structure resists the shear stresses encountered, and the concrete and steel has high compressive strength. Thrust stresses in such a construction are resisted by elongated steel ties extending through the floor between the ends of the arch where it rests on the footings.
A need exists for an earth sheltered structure which has the benefits of the arch form but which may be more easily and relatively inexpensively erected using more or less conventional construction techniques.